Technologies like printed electronics are opening the possibility for light-emitting devices that are light-weight, flexible, low-cost, and relatively efficient compared to, e.g. incandescent light bulbs, and that can be created on large surfaces. Organic light-emitting devices such as the polymer organic light-emitting diode (p-OLED) are slated to revolutionize the use of electronics, bringing it to currently inactive surfaces in applications such as architecture (walls, ceilings, etc.), and consumer packaging (for advertising, information display, etc.). p-OLEDs have the advantage over so-called small molecule OLEDs (sm-OLEDs) in that the polymer active material can usually be manufactured via printing and simple coating techniques, while the small molecule active material in sm-OLEDs require more expensive vacuum processing to manufacture.
OLEDs require two electrodes made of conducting materials to function. In p-OLEDs, the alignment of the work function of the cathode (the negative electrode) with the lowest unoccupied molecular orbital (LUMO) of the light-emitting and semiconducting polymer in the active material, and the alignment of the work function of the anode (the positive electrode) with the highest occupied molecular orbital (HOMO) of the polymer, are critical for the attainment of efficient and balanced charge injection and efficient device operation. Materials that are suitable for the cathode in p-OLEDs in accordance with the above criteria are metals with low work functions (e.g. Ca), and which therefore are highly reactive. Moreover, such low-work function metals are not amenable to solution processing. The necessity for a low-work function metal as one of the electrode materials in p-OLEDs accordingly represents a serious problem from both a stability and fabrication perspective.
Furthermore, at least one of the two electrodes in OLED devices must be transparent; otherwise, the light generated within the active material will never emerge from the device. In OLEDs a transparent and conducting material termed indium tin-oxide (ITO) is commonly used as the anode, but as a material, ITO is not ideal. The surface of ITO is typically very uneven and problems stemming from the formation of hot spots and electrical short circuits during operation of OLEDs are well known in the field. Moreover, the amount of Indium in the world is limited, so the material is becoming increasingly expensive. Finally, devices comprising metals in general, including indium and tin, may be difficult to re-cycle and often require special handling during waste disposal, etc.
To summarize the situation with OLED devices, the current generation is not amenable to solution processing of all components (i.e. the cathode, active material and anode). The active material in sm-OLEDs is typically not amenable to solution processing, and functional p-OLEDs comprise a cathode, which is reactive and not suitable for solution processing. Moreover, typical employed metal-based electrode materials in OLED devices are often expensive, heavy, non-disposable, and in some cases even hazardous
In the latter context, we note that US20090017211A1 discloses how graphene/graphene oxide can be used to replace ITO as the anode material in an OLED, but that no actual functional device is demonstrated.
One way to overcome the drawbacks of OLEDs is to add an electrolyte to the active material to create a device called a light-emitting electrochemical cell, or a LEC. The unique operation of LEC devices is based on mobile ions that are intimately intermixed with the light-emitting (polymer or small molecule) organic semiconductor. These ions redistribute during device operation in order to allow for efficient and balanced electronic charge injection, which in turn eliminates the work function requirements on the electrodes.
Another advantage of LEC devices is that the thickness of the active layer (the inter-electrode gap), in some cases is not critical for the device to operate optimally, as it is in an OLED. This is because the mobile ions in the active material enable electrochemical doping of the organic semiconductor. One example of the thickness independence in LEC devices is described in U.S. Pat. No. 5,677,546, wherein a planar surface cell configuration with a large micrometer-sized inter-electrode gap is disclosed. Moreover, Shin et al. (Applied Physics Letters, 89, 013509, 2006) have demonstrated that planar surface cell devices with an enormous mm-sized inter-electrode gap separating identical (high-work function and stabile) Au electrodes can be operated with efficient emission at a low applied voltage of 5 V.
LECs, however, have other challenges. For example, the mobile ions in LECs enable side-reactions, both electrochemical and chemical. This limits the choice of materials useable in the device, especially for the electrodes. For example, an article describing electrochemical side-reactions associated with using aluminum instead of gold electrodes in LECs were highlighted by Shin et al. (Electrochimica Acta, 52, pp. 6456-62, 2007). The electrochemical stability of the electrode materials is very important in LECs. As a result, no LEC (or OLED) has been demonstrated that solely comprises solution-processable components, i.e. the cathode, active material, and anode, based on metal-free, lightweight and carbon-based materials.
There is hence a need for such a light-emitting device which can be easily produced and which does not have the drawbacks of the prior art devices.